1. Field of the Invention
The present invention relates to a method of calculating pore pressure. The method will calculate pore pressure only looking at one log while drilling, for example resistivity logs.
2. Description of Background Art
FIG. 1 shows an exemplary diagram of a drilling operation. One of ordinary skill in the art will appreciate that the drilling operation shown in FIG. 1 is provided for exemplary purposes only and accordingly should not be construed as limiting the scope of the present invention. For example, the drilling operation shown in FIG. 1 is a seafloor drilling operation, but the drilling operation may alternatively be a land drilling operation.
As shown in FIG. 1, a drilling rig 105 is configured to drill into a formation (e.g., a formation below a seafloor 110) using a drill bit (not shown) coupled to the distal end of a drill string 125. Specifically, the drill bit is used to drill a borehole 130 extending to a target lithology 120. The target lithology 120 may be filled by hydrocarbon or a mineral resource targeted by a drilling operation.
Formations in which pore pressure exceeds hydrostatic pressure at a given depth are referred to as overpressured. The mechanism of overpressure itself does not modify the scope of the present invention.
When drilling in an overpressured formation, the mud weight (i.e., the weight of drilling fluids injected into the borehole) must be high enough to prevent either the pore pressure from moving formation fluids into the borehole in case of high enough permeability formation (e.g., sand) or the pore pressure from breaking down the formation and eventually causing borehole-walls collapse in case of low enough permeability formation (e.g., shale). In the worst case of a high enough permeability formation, formation fluids entering a borehole may result in loss of the well and/or injury to personnel operating the drilling rig. Accordingly, for safe and economic drilling, it is essential that the pore pressure be predicted with sufficient accuracy. In particular, it is beneficial to predict pore pressure pre-drill, i.e., either before any drilling has commenced and/or at a location that the drill bit has not yet reached.
Further, when drilling in overpressured formations, the number of required casing strings (i.e., structural supports inserted into the borehole) may be increased. Specifically, if a sufficiently accurate pre-drill pore pressure prediction is not available, additional casing strings may be inserted prematurely to avoid the possibility of well control problems (e.g., influx of formation fluids, borehole collapse, etc.). Prematurely inserting casing strings may delay the drilling operation and/or reduce the size of the borehole and result in financial loss.
The knowledge of accurate pore pressure is crucial when drilling a well in order to ensure the success of the drilling operation. Pore pressure is also a controlling input parameter in borehole stability modeling, well planning, design, and wellpath optimization.
A problem often encountered when drilling wells in many parts of the world is narrow drilling margins that require great precision in pore pressure prediction in order to prevent any shale instability problem resulting in risk of lost circulation and/or gas kicks/blowouts.
There is a great need in the art for a method that makes it possible to accurately predict pore pressure in real time measurements at the rig site. If such data were available, it would also be useful for identifying high risk shallow water zones, optimizing mud weight, detecting shallow hazard zones, detecting abnormal pressure zones, determining formation strength for wellpath optimization, and, in general, for obtaining the most trouble-free, cost effective drilling.
One of the conventional methods of predicting pore pressure is Eaton's method. Eaton's method involves the following equation:PP=OvB−(OvB−Hyd)F wherein PP is pore pressure, OvB is the overburden value associated with the drilling location, Hyd is hydrostatic pore pressure, and F is (in case of resistivity logs):(R/RE)1.2 wherein R is the measured value of resistivity and RE is the normal compaction trend of resistivity.
The problem with Eaton's method is that the user must pick the correct normal compaction trend, which is sometimes difficult and implies the analysis of offset wells and regional maps. As such, Eaton's method almost never can be applied while drilling, with the knowledge of the real time logs.
Therefore, there is an industry-wide need for a more flexible method of calculating pore pressures.
One of the main bases of the present invention is to properly normalize or scale logs. For example, if we look at the porosity and resistivity logs, and we cross plot these quantities for a number of wells, we may have the plot shown in FIG. 2A. The color code in this case is effective stress, evaluated for each case. Each point in the plot is the average of resistivity and porosity made in only shale lithology for each 20 [m] interval (other values can be 5 [m] or 2 [m] or any other appropriate interval to represent resistivity and porosity there). The shale lithology can be defined through the volume fraction of shale, when this is larger than 65% or 75% or 85% or any other value, larger or lower, that is sufficient to identify a shale. The wells for this case are not compartmentalized. That is, only one pressure gradient exists within the sands in each well. Moreover, all the porosity values larger than 20% are filtered out (typically, these values for shale lithology are in the relatively shallow region), and resistivity of shale close to sand (meaning shale points within 20 [m] or 5 [m] or 2 [m] or any other distance that may affect the resistivity information either because of hydrocarbon leakage in the shale or because of the accuracy/sampling of the resistivity tool) that is not water saturated (for example water saturation less than 85% or 75% or any other percentage that makes the sand not water saturated) is rejected. If a reference of resistivity (Ro) and porosity (øo) is picked for each well in the shale at the beginning of each log (this means that the reference point is at an arbitrary depth at this stage), and the resistivity and porosity for each log are normalized with the reference values Ro, øo (different for each well), a new cross plot can be obtained (FIG. 2B). As evident, all the normalized resistivity logs are following the trend 1/(ø/øo) that is defined here as normal compaction for the section of shale that goes from the reference point up to the end of the log. Moreover, the color code in this case is pore pressure/effective stress. All the wells are following the trend 1/(ø/øo) going deeper with depth, except for the wells where pore pressure is larger (in this example at least 2 times) than the value of effective stress at each depth. When pore pressure is larger than effective stress, in fact, the normalized resistivity and porosity are clustered around the normalized point 1,1 even for deeper depth. This means that if pore pressure is large enough, the porosity versus resistivity does not follow the normal compaction trend 1/(ø/øo), but instead is going to have a larger porosity and lower resistivity compared with the wells where pore pressure is same order with effective stress. Note that the value of 2 is valid for a relatively shallow region. For very deep wells the value or pore pressure/effective stress can increase to 3, 4, or more in order to drive the normalized plot to the position of 1,1. The same analysis could be done cross plotting resistivity vs velocity or other variables. In case the wells are compartmentalized, a more careful selection of the reference depth must be done. For example, with reference to FIG. 2C, the resistivity and porosity in the shale belonging to one compartmentalized well are plotted, all the values being normalize with a resistivity and porosity value in the shale at a depth of reference that in this case is the beginning of the log. The color code in this case is the distance to the reference depth (e.g., deep red color is for depths very close to the reference depth). As evident, no distinct trend can be recognized. However, if, after each compartment has been identified (where each sand lithology has only one pressure gradient), the reference depth is fixed at the beginning of each compartment in the shale region for each compartment (so in this case, there are a different number of reference depths), a clear trend can be identified for each single compartment (FIG. 2D). This is, again, well approximated by 1/(ø/øo). The pore pressure, going from one compartment to another deeper compartment, is increasing (this results is not shown), almost in step, compared with the local effective stress. The expected trend 1/(ø/øo) is a good approximation, especially if the shale interval is short enough and within the same compartment. Even a linear trend between normalized resistivity and porosity can be approximated with 1/(ø/øo) for short intervals. These results give the basis in order to calculate pore pressure.